A well-known ushi-oni is a massive, brutal sea-monster which lives off the coast of Shimane Prefecture and other places in Western Japan and attacks fishermen. It is often depicted with a spider- or crab-like body. This ushi-oni seems to be connected to another monster called the nure-onna 濡れ女 / 濡女 , who sometimes appears before an ushi-oni attack and tricks the victim into holding her child, which then becomes a stone stuck to the person's hands and grows heavier in order to hinder escape.
In Iwami Ginzan (Shimane) the story goes on about a young samurai who held the heavy stone baby.
A famous sword in his family suddenly flew into the sky, cut off the head of the demon and cut the body into nine pieces.

If it rains for a long time in Shimane, the Ushioni starts to show up under bridges. Once a man passed a bridge and saw a white light flickering there. After he had passed the bridge the flickering kept hovering around him. People in the neighbourhood who saw this thought the Ushi-Oni was the reason for it.
Another word for this monster in local dialect is オシミ （牛鬼） oshimi (ushioni) .
The more people try to get rid of the flickering by shaking their sleeves, the more it clings to them. To get rid of it one has to get some urin on the hands and splatter that around.

Kaikidan Ekotoba 怪奇談絵詞Nure-onna 濡女 "Wet Woman" was a dragon with a snake's body and a woman's head.
She was typically seen while washing her hair on a riverbank and would sometimes kill humans when angered.

Once a man went fishing in the evening, when the Nure-Onna appeared. She gave him a baby to hold and disappeared. He threw the baby away and fled, but Ushi-Oni came right after him. He managed to take shelter in a farmer's house and was safe.

Once a brave man was on his way to 伊勢神宮 Ise Shrine with a lot of money for offerings, but an Oni in the form of the 庚申 Koshin Deity took all his money away. But it was just a wicked person clad like Koshin. When the Deity heard the story, he hurried to give the money back to the brave man.

On the evening of Setsubun the 出雲に鬼 demons of Izumo come out. The God of Izumo sits on his bronze horse and drives them away with his bow and arrow.
On this day the people of Izumo do not go outside to see him.
Once a curious old hag peeked out from a small hole, but the Deity saw her and hit her eye with his arrow.

shinba 神馬 the divine horse from Izumo

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那賀郡 Naka district 浅利村 Asari village (Present day 江津市浅利町）

Once upon a time the local Shrine priest went fishing in the evening, when the Nure-Onna gave him a baby to hold. He tried to throw it away, but it stuck to his hands. Finally Ushi-Oni came running after him. The priest run for his life, but suddenly he saw a white flash of light hitting the head of the Ushi-Oni. Later he understood that it was the sword from his Shrine that had come to his rescue.

Once an old man took his small fishing boat and went out at night. An Ushi-Oni came after him, but he caught the demon and tried to birng him to his home. But near his fishing hut the Ushi-Oni fled. A young man who was watching tried to hit the Ushi-Oni and managed to cut off its head. But when they looked closer it was just 椿の古根 an old root of a 椿の古根 tsubaki camellia tree.

- - - Some researchers say the real origin of this monster is a root of a camellia tree (tsubaki no ne).
This tree is sacred in many parts of Japan. Ushi-Oni is in fact a local deity turned demon.
Camellia trees grow in many parts along the beaches and peninsulars, being brought there by the waves of the sea. Camellia flowers blossom at the border to the other world.

Oniko 鬼子 Child of a Demon
The local people here never have a wedding ceremony on the Koshin day.
They say a child conceived on the Koshin day will either become a Demon Child or 蛇の子 the child of a serpent.

- quote - A valley filled with massive, peculiarly-shaped rocks; a reminder of the primeval age
Oni No Shitaburui is a valley in the midstream of Omaki River.
You will never get tired watching the clear water flowing sometimes fast, sometimes slowly, between massive rocks. The valley is surrounded with abundant nature that looks different in different seasons. You can also enjoy the scenery on the walking path.

There is a legend explaining the origin of the name of the place. Once upon a time, a small shark (wani ワニ) fell in love with a princess living in a village, and visited the village every day over the Sea of Japan. The princess felt sick of this and stemmed the water with a large rock to stop the shark, but the shark continued to admire the princess. It is said that the current name is a corruption of wani no shita buru (meaning the shark in love).

On both sides of the valley are steep cliffs, and the rows of massive, peculiarly-shaped rocks are a spectacular sight. There is a dam downstream, and you can enjoy fishing. The barrier-free walking path is suitable for wheelchairs and baby strollers. The Izumo area is a land of myths. Imagine the ancient days in the freely-growing trees forming a scene like a virgin forest of the primeval age. - source : jnto.go.jp/eng/spot/natuscen -

Once upon a time
there lived a wild demon, who was very large and had only one eye. He appeared in the fields and when the farmers were busy, he took away their child and begun to eat it. The farmer and his wife hid in the bamboo forest in great fear and could not do anything to help. They trembled and the bamboo swayed, making quite a sound. When the child heard the sound, it begun to shout "「アヨ、アヨ（動いたよ、動いたよ）」 Ayo! Ayo! (It's moving.)
This is how the region became its name. - reference source : kotodama.日本伝.com -

This legend is mentioned in the Izumo Fudoki:- quote -
the inhabitants of the Central Land who are not subjugated "ashiki," or evil gods. In Izumo fudoki, a one-eyed A appears in a reclaimed land in the community of Ayo of Izumo Province (present-day Shimane prefecture) and devours a man (Akimoto 238-39).
Komatsu Kazuhiko writes,
"People who had different customs or lived beyond the reach of the emperor's control" were considered some form of oni ("Supernatural Apparitions and Domestic Life in Japan" 3). This concept is actually not unique to Japan.
Targets of subjugation and different ethnic groups that do not assimilate the precepts of hegemonic authority are described as %, by the Han race even before the period of Six Dynasties (220-589) in China (Li 427). It is not certain, however, whether the character, was pronounced as oni or mono. Indeed, the character is rendered as mono in Man'yoshu (Ten Thousand Leaves, ca. eighth century).
There exists no definitive example of the term "on" in the ancient literature (Tsuchihashi 94-95). - source : archive.org/stream/JapaneseDemonLore -